The present invention relates to a start-up control for AC motors and more particularly a start-up control for induction motors without tachometer feedback and with arbitrary initial speed.
On initial start-up, and induction motor used in an industrial application can have any speed of rotation within the normal operating range. This could be caused by a previous intentional or inadvertent shutdown. Many induction motors supplied by current controlled or voltage controlled DC to AC inverters do not have tachometer feedback of the rotor speed. Tachometers are high maintenance devices and the motor and drive can be made more rugged and reliable if tachometer feedback is not used. If during an intentional or inadvertent shutdown, motor flux has been lost, it can take a large inertia motor and load minutes to slowly coast to zero speed. If an attempt is made to start the motor while it is rotating without knowing the motor speed, an abrupt change of motor speed could result causing damage to the motor and load.
It is an object of the present invention to provide open loop starting for an induction motor drive rotating at any initial speed.
It is a further object of the present invention to provide open loop starting for an induction motor drive rotating at any initial speed resulting in a smooth transition to closed loop motor operation.